Bridge cranes according to prior art generally comprise two main girders that are at a distance from each other and a trolley comprising the hoisting machinery of the crane, said machinery traveling along with the trolley, and said trolley fitted to travel on rails on top of the girders. One problem, among others, in this solution is the expensive, wide and heavy structure. Two main girders cause both material costs and additional construction-time costs owing to the amount of work needed for manufacturing. Additionally, the trolley must be heavily built and robust in order to withstand the stressing caused by the hoisting machinery. The width of the structure is again a problem in connection with the handling of the material to be lifted. In this case one problem, among others, is that two bridge cranes cannot necessarily be brought close enough together so that large-sized objects could e.g. be turned by means of the cranes.
Also known in the art are bridge cranes which comprise only one main girder and in which the hoisting machinery with its rope drum is rigidly in its position with respect to the main girder. One such structure is presented in British patent no. GB1200075. It presents a single-beam bridge crane, in which both ends of the main girder comprise an end carriage provided with rail wheels, and in which crane the rope drum of the hoisting machinery is fixed into its position on the side wall of the second end carriage. On one side of the main girder is a hoisting trolley, movable in the longitudinal direction of the main girder, to which hoisting trolley the hoisting ropes are guided from the rope drum such that the hoisting rope from the rope drum that is fixed on the first end carriage is guided at first to the diverting pulley on the second end carriage and after passing around it to the hoisting trolley and to the hook and onwards via a diverting pulley on the hoisting trolley back to the first end carriage, to which the free end of the rope is fixed. The rope forces of the hoisting rope that act on the hoisting trolley are compensated in the direction of the hoisting rope and in the lateral direction. Compensation in the lateral direction is implemented such that the diverting pulleys of the hoisting trolley are on the same vertical plane as each other. A problem in this type of structure is the sagging of the long horizontal section of the hoisting rope, which sagging is often disturbingly large from the viewpoint of the user. The reason for the occurrence of the sagging is mainly that the weight of the lifting hook and its rope pulleys is not sufficient to tension the hoisting rope. When the load is fastened to the lifting hook at first in the hoisting phase it always takes the hoisting machinery some time before the slack rope caused by sagging has tightened. This can be dangerous if one cannot be prepared for it, and also in installation work, in which the crane is used for precise lifting, the slackness of the rope caused by sagging is awkward.